-3^ Journal of Comparative Neurology. 



the resulting organ to occupy almost any portion with refer- 

 ence to the axis of the brain. 



The moon-eye, Hyodon, is perhaps the most reptilian 

 brain of the osseous fishes here noticed, and has the simplest 

 cerebellum. It is probably one of the best types in which to 

 study the archetectonic of the cerebellum and optic lobes in 

 comparison with those of reptiles. The optic lobes are large 

 and almost absolutely unmodified by the cerebellum, except 

 in so far as the corpora posteriores are thrust ventrad and 

 laterad. The volvula is well developed, and its dorsal lamina 

 passes directly into the tectum. The valve is small. Im- 

 mediately caudad of the valve the cerebellum swells into a 

 large, thick sac which soon becomes free from the walls of 

 the medulla. The cavity of the cerebellar ventricle is not 

 suppressed, and there is very little to suggest the complicated 

 projections of the calamus and vagus regions of the medulla 

 found in most fishes, especially the carps. The cerebellum 

 thus forms a simple lobe, projecting over the partly exposed 

 opening of the fourth ventricle. It is relatively small, as may 

 be gathered by a comparison with corresponding views of the 

 Gizzard-shad (Plate XIX, Fig. 7). 



As in Lepidosteus, the cephalic part of the mesencephalic 

 ventricle is not entered by the volvula. Transections in front 

 of the oculomotor roots resemble similar sections of reptilian 

 brains, except for the presence of the small hypoaria. The 

 tori longitudinalis, or appendages of the median line of the 

 tectum, are large. 



The volvula extends cephalad to just in front of the third 

 nerve-roots where it appears two-lobed, having the appar- 

 ently ectal (ventricular) surface composed of gray matter, a 

 median zone of Purkinje's cells, and a central mass of white 

 matter. Farther caudad the cavity due to the invagination 

 is encountered, surrounded by the white zone, which is, 

 therefore, morphologically ectal. The dorsal wall of the sac 

 thins out and passes into the membranous caudal wall of the 

 optic lobes, which are obviously paired at this level. 



